Photographs that appear to identify one of the grown-up killers of James Bulger have been removed from the internet after the attorney general, Dominic Grieve, launched an investigation.

The pictures purported to show Jon Venables, now 30, who was given a new identity following his initial release from prison. Venables and his classmate Robert Thompson abducted and murdered two-year-old James in Liverpool 20 years ago this week.

A court order made by Dame Elizabeth Butler-Sloss of the high court family division is still in force, banning publication of any information that could lead to revelation of their new identities.

The online pictures appeared on Twitter and claimed to show Venables as an adult posing with friends. The Twitter user has since removed the posts.

A spokesman for the attorney general's office said: "The AGO has been alerted to a possible contempt of court. We are liaising with the ministry of justice and others to establish the facts.

"We can neither confirm nor deny whether the pictures in question are of Jon Venables. It should be noted there is a worldwide injunction in place which prevents the publication of any images or information purporting to identify anyone as Jon Venables."

James Bulger's mother, Denise Fergus, has made no comment about the alleged breach. In the past she has opposed the injunction, fearing it could lead to innocent men being accused of being James's killers.

Niri Shan, head of media law at law firm Taylor Wessing, suggested the attorney general should use the case to send a message.

"As highlighted by [the director of public prosecutions] Kier Starmer's recent guidelines on social media, people are subject to the same laws of contempt as the mainstream media and publishing a photo of Venables, in breach of the court order, is clearly contempt of court," Shan said.

"Whether the attorney general decides to prosecute will most likely depend on number of followers the person who tweeted the photos has. This would be a good case for him to send a strong message to those that use Twitter that they breach court orders at their own peril."